Three years ago, somewhere near this dusty little town of watermelon fields and senior citizen trailer parks, a pudgy, prolific science fiction writer named L. Ron Hubbard climbed into a black van and reportedly disappeared from sight.

Nobody in Hemet, 80 miles east of Los Angeles, or anywhere else might have cared about the fate of a 71-year-old eccentric with a lust for privacy, except that Hubbard was the founder of one of the word's wealthiest and most controversial new religions.

A brilliant organizer, he had turned a talent for amateur psychotherapy into the Church of Scientology, a $300 million, 2 million-member ideological and administrative colossus.

Since Hubbard's alleged disappearance, his church has been riddled with high-level defections and lawsuits. His wife and several top church officials were imprisoned on various charges stemming from 1977 raids of the church's Washington and Los Angeles offices, including conspiracy to obstruct justice or to bug and burglarize the Internal Revenue Service and the Justice Department.

And now a comic-opera legal fight has ensued to prove or disprove his existence through tape recordings, fingerprinted letters and special inks.

Dead or alive, Hubbard's reluctance to deal directly with the outside world still influences somewhat his church and its widespread properties. The church's Golden Era Studios, a resort-like film-making complex at the foot of the San Jacinto Mountains six miles north of here, is surrounded by black metal fences with card-slot locks at each entrance. A recent unannounced visitor was photographed by security men and gently encouraged to move on.

Church officials say they have not seen nor spoken to Hubbard for years but are confident he is alive and well. A four-page letter in a nearly illegible hand, but stamped with fingerprints to prove it was Hubbard's work, recently reached his lawyers in a Federal Express envelope with no return address.

Denver's Rocky Mountain News recently reported that Hubbard had given it an interview, ending 15 years of refusing to speak to the press, but the interview consisted of written answers to questions submitted through Hubbard's lawyers. His attorneys say they must keep confidential whether they even know where he is.

Vaughn Young, a writer and Scientology Church member who is completing a biography of Hubbard, said his subject has enjoyed long periods of solitude for most of his life. "Even in college, his professors had trouble finding him," Young said. He said there is little or no connection between this habit and the recent troubles of the church.

If the mystery of L. Ron Hubbard is ever solved, it may happen not far from here, in the Riverside County Courthouse where Hubbard's estranged eldest son has asked to be appointed trustee of his father's estate on the grounds that Hubbard is dead or missing. Court papers do not place a value on the estate, but one former Hubbard associate said it is worth at least $100 million.

With less than filial kindness, Ronald E. DeWolf, 48, who changed his name from L. Ron Hubbard Jr. in 1972, alleged in court papers that his father "has lived a life characterized by severe mental illness and physical disease, consistent failure, and the use of false and fraudulent, oftentimes criminal means, to cover up these failures and to acquire wealth, fame and power in order to destroy his perceived 'enemies.' "

Barrett S. Litt, a Los Angeles attorney, called the allegations untrue and said the judge in the case had struck them from the record as "scandalous and irrelevant." Litt said he is representing Hubbard's third wife, Mary Sue, now in federal prison after being convicted of obstruction of justice, in her effort to have DeWolf's petition rejected.

The letter that Hubbard's attorneys recently received ignored most of DeWolf's charges.

"Ron DeWolf was a war baby," said the letter. "I was never there. His mother was an alcoholic and deserted me at war's end when the allocation from the govt. ceased and I was in the hospital at war's end, the usual wounded veteran's story. She ran off with the children and another man. It's too bad I never had the opportunity to raise him during his formative years. Had I been able to do so he might have turned out differently."

The letter also said that Hubbard was well, that his estate and business affairs were being competently handled and that his son "is not in a position to know about me or the church or my activities." (DeWolf left the church in 1959 and hasn't seen his father since then. He manages an apartment building in Carson City, Nev.)

Four recognized experts have submitted court declarations verifying the handwriting and fingerprints as Hubbard's. But Los Angeles attorney Wilkie Cheong, representing DeWolf, called it just "a document with ink and fingerprints. Legally it has no value." He also is attempting to determine the validity of a tape recording of Hubbard's voice which has been submitted as further proof of Hubbard's existence. To dismiss the action, Cheong said, Hubbard should appear in court, if he can.

Hubbard's attorneys, Sherman and Stephen Lenske, called the letter "an important piece of evidence." Sherman Lenske said experts told him they could determine if prints came from a dead man because a body decomposes rapidly. One of the experts who validated Hubbard's letter, retired U.S. Treasury fingerprint and document expert Howard C. Doulder, said he probably could not tell the difference if fingerprints were from a carefully preserved body, but added that he had been shown "boxes and boxes" of recent manuscripts in Hubbard's handwriting and was certain he was alive.

So where is Hubbard? Doulder said the recent dates on the documents from Hubbard indicated he was somewhere in the United States, perhaps still in California. Hubbard's son, DeWolf, said, "I think he's dead and has been for some time."

DeWolf cited his father's failure to contest personally a $12 million suit filed in Boston federal court filed by author Paulette Cooper, who said she was mercilessly harassed after writing a book critical of Scientology. The judge recently issued a default order against Hubbard.

"You're talking about something that is very near and dear to my father's heart, which was money," DeWolf said.

Boston attorney Michael Flynn, representing author Cooper and other church adversaries, said, "I think he Hubbard is alive and in hiding."

Hubbard founded Scientology in 1954, based on a form of psychotherapy he had invented called "Dianetics." The core of the religion is an "audit" in which individuals confess painful or embarrassing moments from their past while "on the cans," a reference to a lie detector device which operates while the subject holds two tin cans. A counselor uses the measurements of emotional distress to help the individual overcome negative feelings which have made him unhappy and unproductive, according to church members.

Fees for audits can run as high as $300 an hour. The church's books, films and other counseling systems have raised millions of dollars, which now interests both the IRS and many people who say they have been injured by church-organized harassment campaigns.

Hubbard's biographer, Vaughn Young, said a new group of church leaders has removed those responsible for the bugging and other mistakes.

The Lenskes, although not church members, said they have come to admire the Scientologists thay have met. Last year, they said, the church experienced significant growth in membership despite 30 or 40 defections.

One recent defector, former executive director Bill Franks, said he still admires Hubbard and feels the audits have helped many people. DeWolf said he is suspicious of the use of the audits.

The most recent Hubbard letter, if authentic, indicates that the church's founder is oblivious to much of the controversy. He says he has not been an officer of the church for "nearly 17 years." He mentions his new novel, "Battlefield Earth," his new "Space Jazz" album and a nearly completed 10-volume novel called "Mission Earth."

"I am and always have been a writer and, as a writer to do one's job one can't be involved in the constant noise and hurley burly of distracting things," the letter said. "So to complete my contracts it was vital I sat down under the big trees and let the rest of the world go by."